Automatic retransmission request control system and retransmission method in MIMO-OFDM system

ABSTRACT

An automatic retransmission request control system in an OFDM-MIMO communication system includes a retransmission mode selection part which selects a retransmission mode from among (a) a mode in which to transmit the data, which are to be retransmitted, via the same antenna as in the previous transmission, while transmitting, at the same time, new data by use of an antenna via which no data retransmission is requested; (b) a mode in which to transmit the data, which are to be retransmitted, via an antenna via which no retransmission is requested, while transmitting new data via another antenna at the same time; (c) a mode in which to use STBC to retransmit the data via an antenna via which no retransmission is requested; and (d) a mode in which to use STBC to retransmit the data via all the available antennas.

TECHNICAL FIELD

The present invention relates to an automatic repeat request (ARQ)control system and a retransmission method in a multiple-inputmultiple-output (MIMO) communication system that employs orthogonalfrequency division multiplexing (OFDM).

BACKGROUND ART

Simultaneous transmission of multiple data streams is carried out in aMIMO communication system that employs multiple (NT) transmissionantennas and multiple (NR) receiving antennas. Depending on the usage,MIMO system contributes to improvement of performance by spatialdiversity or contributes to increase of system capacity by spatialmultiplexing. The presence of random fading and multipath delay spreadin a wireless communication system enables such improvements.

The multiple communication channels present between the transmissionantennas and receiving antennas usually change with time and havedifferent link conditions. MIMO systems having feedback provide thetransmitter with the channel state information (CSI), allowing the useof methods such as link adaptation and water filling to provide a higherlevel of performance.

A well-known technique to increase data rate by spatial multiplexing isdiscussed in Non-Patent Document 1.

Spatial diversity is implemented by space-time block coding, whichprovides the full advantage of diversity. The space-time block code isdisclosed, for example, in Non-Patent Document 2.

MIMO techniques were first designed assuming a narrowband wirelesssystem, namely a flat fading channel. Therefore, it is difficult toachieve high effects in frequency selective channels. OFDM is used inconjunction with MIMO systems to overcome the frequency selectivechannels proposed by the wireless environment.

OFDM is capable of converting the frequency selective channel into a setof independent parallel frequency-flat subchannels using the inversefast Fourier transform (IFFT). The frequencies of these subchannels areorthogonal and mutually overlapping, thereby improving spectralefficiency and minimizing inter-carrier interference. Attaching a cyclicprefix to the OFDM symbol further reduces the multipath effects.

With future technology shifting to accommodate a high speed service withincreased IP dependency, it is necessary to meet requirements such asspectral efficiencies, system user capacity, end-to-end latency, andquality-of-service (QoS) management. While MIMO-OFDM systems meet someof these criteria, ARQ techniques also play an important role inensuring fast and reliable delivery.

ARQ is a technique for transmitting a retransmission request forreceived packet data upon detecting an error in the received packetdata. With the transfer of a large volume of high-speed data, moreefficient ARQ techniques are typically used to reduce the number ofretransmission requests.

It is obviously shown that Hybrid ARQ (HARQ) techniques include chasecombining and incremental redundancy and improve efficiency by reducingARQ overheads. HARQ techniques are primarily designed assuming asingle-antenna transmitter and receiver.

-   Non-Patent Document 1: V-BLAST: an architecture for realizing very    high data rates over the rich-scattering wireless channel” by P W    Wolniansky et al in the published papers of the 1998 URSI    International Symposium on Signals, Systems and Electronics, Pisa,    Italy, Sep. 29 to Oct. 2, 1998.-   Non-Patent Document 2: Tarokh, V., Jafarkhani, H., Calderbank, A.    R.: Space-Time Block Codes from Orthogonal Designs, IEEE    Transactions on information theory, Vol. 45, pp. 1456-1467, July    1999, and in WO 99/15871.

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

However, no technique has been disclosed where HARQ is applied toMIMO-OFDM systems.

In the light of this fact, the present invention has been made, and itis therefore an object of the present invention to provide an automaticrepeat request control system and a retransmission method capable ofcontrolling the retransmission methods according to various systemrequests when HARQ is applied to MIMO-OFDM systems. Further, it is anobject of the present invention to achieve improvement of datathroughput performance by improving accuracy in the retransmission ofsignals and reducing the number of retransmission requests.

Means for Solving the Problem

The automatic repeat request control system and retransmission method ina MIMO-OFDM system according to the present invention comprise thefollowing configurations and steps.

An ARQ controlling section module at the transmitter determines whetheror not retransmission of signals is required. The module also controlsthe types of usage schemes when retransmission is required. The ARQmodule makes decisions based on ARQ feedback information from thereceiver. The system requirements such as whether or not the systemallows an error and whether or not the system allows delay also play animportant role in the decision process.

In the system of the present invention, the receiver carries out cyclicredundancy checks (CRC) for antenna chains and the transmitterdetermines whether or not data retransmission is required fortransmission antennas by feedback information of acknowledgement (ACK)or negative acknowledgment (NACK) based on a result of CRC from thereceiver.

When retransmission is required, the retransmission is carried outaccording to one of the four retransmission schemes the presentinvention proposes. An optimal retransmission scheme is selected, basedon different system requirements criteria such as latency andperformance level.

In the present invention, antennas that receive ACKs are considered tobe more reliable than antennas that receive NACKs. Data transmitted byreliable antennas has a higher probability of recovering data correctly.

In one embodiment of the present invention, data for retransmission istransmitted using the same antennas as previous transmission while newdata is transmitted using antennas without retransmission requests. Thismethod provides an advantage of reducing complexity accompanying withdata retransmission and improving efficiency.

In another embodiment of the present invention, data for retransmissionis transmitted using the reliable antennas (antennas withoutretransmission requests), and new data is transmitted using otherantennas. This method provides an advantage of reducing the number ofretransmissions required for a certain error packet as well as adrawback of increased complexity.

In another embodiment of the present invention, data retransmissionusing STBC that is the spatial diversity technique is carried out usingreliable antennas. By this method, it is possible to respond to arequest even in a system that does not allow error required for accurateretransmission scheme having less delay.

In a further embodiment of the present invention, STBC is also used fordata packet retransmission, but retransmission is performed using notonly reliable antennas, but also all available antennas. This method isused as the most accurate retransmission scheme and is appropriate for asystem that does not allow an error, but allows delay.

A variation of the embodiments using STBC for retransmission uses ahigher order of modulation for improved retransmission efficiency.

Variations of the above embodiments include the use of IncrementalRedundancy (IR) type of ARQ and retransmission using various sets ofinterleaving patterns to improve system performance.

A further variation of the above embodiments includes link adaptationusing long-term ARQ statistical information. In this case, it is notnecessary to perform feedback of channel status information (CSI) andcomplicated processing.

Effect of the Invention

According to the present invention, it is possible to control theretransmission method according to various system requests when HARQ isapplied to MIMO-OFDM systems. Moreover, according to the presentinvention, it is possible to achieve improvement of data throughputperformance by improving accuracy in the retransmission of signals andreducing the number of retransmission requests.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of the transmitter for the MIMO-OFDMcommunication system;

FIG. 2 is a block diagram of the receiver for the MIMO-OFDMcommunication system;

FIG. 3 is a diagram of the functional blocks of the transmission andreceiving ARQ controlling section of the present invention;

FIG. 4 shows an example of scenario for the retransmission setting ofdata packets by a retransmission method of the present invention;

FIG. 5 shows an example of scenario for the retransmission setting ofdata packets by a retransmission method of the present invention;

FIG. 6 shows an example of scenario for the retransmission setting ofdata packets by a retransmission method of the present invention; and

FIG. 7 shows an example of scenario for the retransmission setting ofdata packets by a retransmission method of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, embodiments of the present invention will be described in detailwith reference to the drawings.

FIG. 1 is a diagram of transmitter 100 for a multiple-inputmultiple-output communication system that utilizes orthogonal frequencydivision multiplexing (namely, a MIMO-OFDM system). FIG. 2 is a diagramof receiver 200 of the same system. Although both figures show thesystem employing two transmission antennas and two receiving antennas,the present invention can be extended to a system for employing multiple(NT) transmission antennas and multiple (NR) receiving antennas.

At transmitter 100, data processing is performed for each individualantenna chain. Different streams of independent data are transmittedfrom the individual transmission antennas. The input data is firstattached the cyclic redundancy check (CRC) code at CRC attaching section102. Then, channel coding such as convolutional coding and turbo codingis carried out at coding section 104. The coded data will then beinterleaved by interleaver 106 to reduce burst errors in the data. M-arymodulation constellation symbol mapping is executed on the interleaveddata at mapping section 108. A pilot signal is inserted in the mappedsignal at pilot inserting section 110. Pilot signal insertion makeschannel evaluation at the receiver straightforward.

Before carrying out OFDM modulation, the serial data stream is convertedinto parallel data streams by S/P converting section 112. IFFT section114 causes the generated sub-carriers mutually orthogonal. After theparallel data is converted into serial data by P/S converting section116, a cyclic prefix for reducing multipath effects is attached to theOFDM symbol by CP attaching section 118. Prior to transmission, thedigital signal is converted to analog signal by D/A converting section120. After the various processes in each transmitter chain, signalsbecome available for transmission through the allocated transmittingantennas 122.

At receiver 200, the reverse processes such as conversion from analog todigital (A/D converting section 204), removal of cyclic prefix (CPremoving section 206) and serial parallel conversion (S/P convertingsection 208) fast Fourier transform (FFT section 210) and parallelserial conversion (P/S converting section 210) are carried out for thereceived signals from receiving antennas 202. The received signals arecomprised of overlapping signals from a plurality of transmissionantennas, and it is therefore necessary to separate the signals into theindividual streams. In this case, V-BLAST decoder 214, which utilizeszero forcing (ZF) or minimum mean square error (MMSE) techniques, isused to perform this function.

After carrying out demapping (demapping section 216), deinterleaving(deinterleaver 218) and decoding (decoding section 220), cyclicredundancy check (CRC processing section 222) is then performed on eachpacket to validate the data. If it is determined that the checked packetdoes not include error, acknowledgment (ACK) is transmitted to thetransmitter and the transmitter does not retransmit the packet. If thereis an error, a negative acknowledgment (NACK) is transmitted totransmitter 100 for retransmission request.

FIG. 3 is a diagram of the functional blocks of the transmission andreceiving ARQ controlling section for the present invention.

As illustrated in FIG. 3, each antenna chain will have its dedicated CRCattaching section 302. Therefore, at ARQ controlling section 316 of thereceiver, every data packet on each individual receiving antenna chainwill undergo CRC for error detection in CRC processing section 318. Thereceiver will then feedback ARQ information related to each of the datastreams from ACK/NACK output section 320 via fast ARQ feedback channel322 to a plurality of ACK/NACK receiving sections 308 at ARQ controllingsection 306 of the transmitter. This configuration provides an advantageof not requiring data retransmission from all antennas when an error isdetected. Only the corrupted data streams require a retransmission. Theprobability of all data streams having errors is low, and thistransmission method leads to an improvement in the data throughput.

Based on the ARQ information obtained at ACK/NACK receiving section 308,error data stream detecting section 310 specifies data streams thatrequire retransmission. Furthermore, error data stream detecting section310 stores the long-term statistics of ARQ performed, namely the averagenumber of retransmissions occurred at the specific transmission antenna.This information is utilized in the process of M-ary modulation andcoding at AMC section 304. For example, if the number of retransmissionsas the long-term ARQ statistics for a transmission antenna is smallerthan that for other transmission antenna, a higher order of modulationis set at the transmission antenna. To the contrary, for a transmissionantenna having a greater number of retransmissions compared to othertransmission antenna, a lower order of modulation is set.

If retransmission is required, the retransmission mode selecting section312 will execute decision process of selecting the appropriate scheme touse for data retransmission. Transmission buffers 314 is updatedaccordingly.

FIG. 4 to FIG. 7 show the examples of scenarios for the four differentretransmission methods proposed in the present invention. The methodswill be described below in detail. Each method is suitable for adifferent set of system requests. Therefore, the method to be executedis a method that is most likely to optimize system performance accordingto the user requests.

FIG. 4 and FIG. 5 depict examples of the previous transmission statusand the current retransmission arrangement for methods I and II. Bothmethods retransmit data only for the corrupted streams whilesimultaneously transmit new data for antennas which are not used forretransmission purposes. These methods provide advantage of transmittingnew data continuously even when retransmission is occurred. Therefore, aconsistent level of data rate is maintained without wasting the requestfor accuracy.

In one embodiment of the present invention, as shown in FIG. 4, packets1 to 4 are transmitted on each of the transmission antennas. Based onthe ACK and NACK information from the receiver, packets 2 and 4 arefound to have an error. For retransmission using method I, data to beretransmitted, namely packets 2 and 4, are transmitted using the sameantennas as before. New data is transmitted on antennas withoutretransmission requests.

In the case of retransmission using method II, retransmission data istransmitted using not the same antenna, but the antenna where the errordid not occur at the previous transmission. By transmittingretransmission data through antennas that are considered to be morereliable, retransmission data is likely to have no error, therebyincreasing the data accuracy. The assumption of an antenna beingreliable if an ACK is received for that particular antenna is applied toa stable environment where fading is slow or static.

The above two methods have a difference in antenna allocation. In methodI where allocation is not performed, data processing kept to a minimum,thereby reducing complexity. Therefore, the processing delay in thiscase will be short. For method II, attention has to be put to bothtransmission and receiving buffers due to the changed data setting. Thetransmitter needs to inform the receiver of the difference inarrangement between previous and current transmissions so that thebuffers can be properly updated. This notification from the transmitterto the receiver is regulated by an upper layer. This method II aims atimproving the accuracy of retransmission to reduce the number ofretransmissions requested for a data frame.

Methods I and II is useful to systems which allow error. For suchsystems, transmission of a large volume of data in a short time isrequired while the accuracy follows next. Some examples of suchapplications include video streaming and facsimile. Compared to methodI, method II is suitable for systems which do not allow delay.

On the other hand, when the system does not allow error, methods III orIV is more suitable. In this case, obtaining a right accuracy is giventhe highest priority. These applications include e-commerce, webbrowsing, email access and other interactive services such as instantmessaging.

FIG. 6 and FIG. 7 show examples of the previous transmission status andthe current retransmission arrangement for methods III and IV. Data isretransmitted using space-time block coding (STBC) that is the spatialdiversity technique for higher accuracy of retransmission data. In bothmethods, new data transmission does not occur simultaneously with thedata retransmission. If retransmission is requested, antennas will beused for this purpose only.

In another embodiment of the present invention, as shown in FIG. 4C,retransmission of data packets using method III is carried out on thosereliable antennas (antennas where ACKs are received in the previoustransmission). Transmissions do not occur for the rest of the antennas.

In a further embodiment of the present invention using method IV, datato be retransmitted is transmitted using STBC on all available antennas.Therefore, the probability of error at the receiver is greatly reduced.

For both methods III and IV, in the example where two data packets needto be retransmitted, packet 2 is retransmitted at the first slot whilepacket 4 is retransmitted at the next slot. One way of improving theefficiency is to use a higher order of modulation so that theretransmission data rate can be improved. More retransmission data canbe transmitted at the same instance if this solution is employed.

Unlike method IV, method III aims at reducing the time for processing atthe receiver. Usage of fewer transmission antennas makes the decodingfor STBC straight forward and fast. Furthermore, by retransmitting onreliable antennas, method III attempts to achieve a balance betweencomplexity and accuracy. Although method IV is more complicated andtakes more time, a higher accuracy of retransmission is obtainedcompared to method III. Therefore, method III is suitable for systemwhich allows not error, but delay.

One aspect of this present invention is that selection of methods mayvary according to the performed retransmissions. This is because thesystem requests may change after a certain number of retransmissions ofthe same data packet. For instance, system which allows error selectsmethod I or II for retransmission. However, after a couple ofretransmissions, the same data packet is still in error. Hence, toimprove the accuracy of that packet, retransmission mode selectingsection 312 may decide on a more accurate method III or IV. Instructionsto switch retransmission methods are regulated by the upper layer.

A variation on the above embodiments is to employ ARQ using incrementalredundancy instead of simple chase combining. Incremental redundancyinformation is transmitted in a retransmission packet for furtherimprovement of performance during decoding process.

As another variation on the above embodiments, an interleaving patternmay be employed at retransmission. OFDM sub-carriers may experiencedifferent fading. When channel state information (CSI) is present, bitloading may be performed. For the present invention where CSI is notobtained at the transmitter, equal bit loading is employed. To utilizethe sub-carrier fading differences, interleaving pattern is varied foreach retransmission to balance the effects of fading.

In a further variation on the above embodiments, adaptive modulation,coding and power control may be employed concurrently with the presentinvention. Information obtained from long-term statistics of ARQ ishelpful in identifying those reliable antennas. Those antennas having alow average rate of retransmissions is considered to be reliable. Ahigher order of modulation or a higher rate of coding can be employed onsuch antennas, whereas higher power can be applied to the other antennasto make signal strength higher. Using ARQ statistics as controlinformation instead of the conventional use of CSI for link adaptationis useful for a method that is not complicated and does not take muchtime in determining the differences in link quality.

The above description is considered to be the preferred embodiment ofthe present invention, but the present invention is not limited to thedisclosed embodiments, and may be implemented in various forms andembodiments and that its scope should be determined by reference to theclaims hereinafter provided and their equivalents.

INDUSTRIAL APPLICABILITY

The present invention is suitable for using a multiple-inputmultiple-output (MIMO) communication system employing orthogonalfrequency division multiplexing (OFDM).

The invention claimed is:
 1. A retransmission method based on automaticrepeat request (ARQ) in a multiple-input multiple-output (MIMO)communication system using orthogonal frequency division multiplexing(OFDM), the system comprising a receiver and a transmitter each having aplurality of antennas, the method comprising: at the receiver: executinga cyclic redundancy check (CRC) for data received from the transmitter,for every one of the antennas of the transmitter; and transmitting tothe transmitter feedback information of acknowledgment (ACK) or negativeacknowledgement (NACK) based on a result of the cyclic redundancy check;and at the transmitter: identifying a data stream that requiresretransmission based on the feedback information; and selecting aretransmission mode based on (i) the feedback information for the datastream which requires the retransmission as indicated by the feedbackinformation corresponding to the negative acknowledgment (NACK) and (ii)a type of application requiring the data stream, wherein: (a) theselecting of the retransmission mode selects, if the applicationrequiring the data stream is video streaming or facsimile, a mode ofretransmitting the data which requires the retransmission as indicatedby the feedback information corresponding to the negative acknowledgment(NACK) and simultaneously transmitting a new data; and (b) the selectingof the retransmission mode selects, if the application requiring thedata stream is e-commerce, web browsing email access or instantmessaging, a mode of retransmitting the data which requires theretransmission as indicated by the feedback information corresponding tothe negative acknowledgment (NACK) using a space-time block coding(STBC).
 2. The retransmission method according to claim 1, wherein theidentifying of the data stream further comprises storing ARQ statisticsfor each transmitted data stream and making link adaptation decisionsusing the ARQ statistics.
 3. The retransmission method according toclaim 2, wherein the link adaptation comprises adaptive modulation andcoding and power control.
 4. A system for automatic repeat request (ARQ)control in a multiple-input multiple-output (MIMO) communication systememploying orthogonal frequency division multiplexing (OFDM), the systemcomprising: a receiver and a transmitter each having a plurality ofantennas, the receiver comprising: a CRC processing section thatexecutes a cyclic redundancy check (CRC) for data received from thetransmitter, for every one of the antennas of the transmitter; and anACK/NACK output section that sends to the transmitter feedbackinformation of acknowledgement (ACK) or negative acknowledgment (NACK)based on a result of the cyclic redundancy check; and the transmittercomprising: an error data stream detecting section that identifies adata stream that requires retransmission based on the feedbackinformation; and a retransmission mode selecting section that selects aretransmission mode based on (i) the feedback information forretransmitting the data stream which requires the retransmission asindicated by the feedback information corresponding to the negativeacknowledgment (NACK) and (ii) a type of application requiring the datastream, wherein: (a) the retransmission mode selecting section selects,if the application requiring the data stream is video streaming orfacsimile, a mode of retransmitting the data which requires theretransmission as indicated by the feedback information corresponding tothe negative acknowledgment (NACK) and simultaneously transmitting a newdata; and (b) the retransmission mode selecting section selects, if theapplication requiring the data stream is e-commerce, web browsing emailaccess or instant messaging, a mode of retransmitting the data whichrequires the retransmission as indicated by the feedback informationcorresponding to the negative acknowledgment (NACK) using a space-timeblock coding (STBC).
 5. The system according to claim 4, furthercomprising a multiple ACK/NACK receiving section that collects ARQstatistics obtained from the receiver.